The most beautiful part of the del Norte is from Irun to Bilbao, which takes about 7 days. (It is also the most strenuous, but I was pretty out of shape when I started and it did not kill me.) The food is also much better in the Basque Country than elsewhere on the del Norte. If you only have 12 days, you could do that week, and then fly or take a train from Bilbao to somewhere close to Santiago (100Km out would still get you a compostela, if that is important for you - I would recommend starting in Lugo, which is an impressive Roman city on the Primitivo.)
If you are interested in walking continuously to Santiago, I would recommend doing the Primitivo, which has a lot of beautiful scenery. If you are fast walkers, you could start in Oviedo (also a beautiful small city) and make it to Santiago in 12 days; if you are slower walkers you would want to start a bit further along the camino. Alternatively, you could do the Portuguese from Porto, which only takes about ten days, so you would have a day each to explore Porto and Santiago.